1. Field of the Invention
The present invention relates to a vehicle operator protection system such as an air bag system and more particularly to a system for determining the existence of an abnormality in a backup circuit for supplying power to the actuating device for actuating the protection system.
2. Description of the Prior Art
In a conventional air bag system, a chargeable device (for example, backup capacitor) is installed in parallel with an actuating device of the air bag system to insure that the air bag system would be actuated even if the battery of the vehicle becomes inoperable due to a collision or the like.
The backup capacitor is charged by the battery while the battery is operating normally, and when the battery becomes inoperable in a collision or the like, the backup capacitor in place of the battery may energize the actuating device by discharging the electric charge that has been stored therein while the battery was operating normally.
Therefore, it is necessary to determine the value of the static capacitance of the backup capacitor to determine whether or not the capacitor it can store the amount of charge necessary to actuate the actuating device.
In order to make that determination, Japanese patent publication No. 44912/1980 discloses that (1) the period for the backup capacitor to attain a predetermined voltage is compared with the required amount of time that the backup capacitor must be connected to the battery, or (2) the time-charging voltage characteristics diagram of a circuit having a standard time constant is compared with the time-charging voltage characteristics diagram of the circuit having a particular time constant.
According to the prior art, a deterioration either in the backup capacitor itself, or in the performance of the charging of the backup capacitor (for example, performance deterioration due to increase of leakage resistance) can be determined.
However, the purpose of the backup capacitor is to maintain a voltage sufficient to actuate the air bag device, even when the battery fails for any reason or the battery terminal becomes disconnected. Therefore, it is necessary to determine if there is any abnormality in the static capacitance of the backup capacitor that would prevent it from storing a voltage large enough to actuate the air bag system reliably.
When the backup capacitor is charged by the battery voltage and the time necessary to attain a predetermined voltage is compared to a required time as in the prior art, because the backup capacitor connected to the battery is susceptible to variation of the battery voltage, high precision abnormality determination of the static capacitance of the backup capacitor can not be attained.
Also, when a variation of charging voltage in a standard time constant circuit is compared to a variation in charging voltage of the backup capacitor, special expensive elements are needed, as it is necessary to use relatively new elements operating at low temperature in order to prepare the standard time constant circuit.